1. Field of the Invention
The present invention relates to an exposure mask used for the photolithography process as one of the semiconductor device fabrication processes. More particularly, the invention relates to a Levenson-type phase-shifting mask that suppresses effectively the optical proximity effect to thereby improve the resolution, and a method of forming a pattern using the mask.
2. Description of the Related Art
In recent years, high-speed operation and large-scale integration of semiconductor devices have been progressing further. According to this tendency, it has been required to further miniaturize the patterns of layers that form the devices.
More recently, the design rule has been decreased to approximately half of the wavelength of exposure light (i.e., exposure wavelength). Thus, it is extremely difficult to form small patterns with the size of approximately half of the exposure wavelength by using ordinary exposure methods. To cope with this, various types of xe2x80x9csuper-resolving techniquexe2x80x9d have been developed and discussed.
One of the known xe2x80x9csuper-resolving techniquesxe2x80x9d is the xe2x80x9cphase-shifting maskxe2x80x9d. This mask is an exposure mask having a patterned phase-shifting layer selectively formed on the transparent parts (e.g., openings) of the transparent substrate. The patterned phase-shifting layer eliminates the effect of diffraction of exposure light passing through adjoining transparent parts, thereby raising the resolution of the mask.
The xe2x80x9cLevenson-typexe2x80x9d phase-shifting mask provides much enhancement of the resolution, which is disclosed, for example, in the Japanese Examined Patent Publication No. 62-50811 published in 1987. With the xe2x80x9cLevenson-type phase-shifting maskxe2x80x9d, a patterned phase shifting layer is alternately formed on adjoining transparent parts of the transparent substrate. This is to make the light beams passing through the transparent parts opposite in phase to each other, thereby suppressing the interference between these two beams. In this way, the mask enhances its resolution.
The xe2x80x9cLevenson-typexe2x80x9d phase-shifting mask is very effective to enhancement of the resolution and the depth of focus for periodically-arranged pattern elements. This mask can resolve extremely miniaturized patterns with the size of approximately half of the exposure wavelength or less. Therefore, it has been thought that this type of mask is most hopeful as the technique that realizes formation of patterns with the size of approximately half of the exposure wavelength or less.
FIG. 3 shows an example of circuit or element patterns (hereinafter, which are referred as circuit/element patterns) to be formed. In FIG. 3, the circuit/element pattern 110 is used to pattern a conductive film formed on a gate dielectric film, thereby forming the gate electrodes of Metal-oxide-Semiconductor Field-Effect Transistors (MOSFETs) and the wiring lines connected thereto. The pattern 110 is made of any photoresist film.
The circuit/element pattern 110 includes an isolated pattern section 113 with an isolated, L-shaped pattern element 113a and a periodic pattern section 114 with closely-arranged, linear pattern elements 114a. The isolated pattern section 113 includes the L-shaped pattern element 113a only, in which no other pattern elements are located near the element 113a. The periodic pattern section 114 includes the linear pattern elements 114a that are arranged in parallel at equal spaces or intervals, which is termed the Line and Space (L/S) pattern.
Actually, the circuit/element pattern 110 of FIG. 3 includes various types of other pattern elements than the elements 113a and 114a. However they are omitted in FIG. 3 for the sake of simplification of explanation.
FIG. 1 shows a prior-art Levenson-type phase-shifting mask used to foam the circuit/element pattern 110 of FIG. 3. FIG. 2 shows a prior-art ordinary (e.g., non-phase-shifting) mask (not the Levenson-type) used to form the same pattern 110.
The prior-art phase-shifting mask 120 of FIG. 1 is of the positive type. The mask 120 comprises an L-shaped blocking or light-shielding part 122a for forming the pattern element 113a of the pattern section 113 of the pattern 110 in FIG. 3 and six linear blocking parts 122b for Forming the pattern elements 114a of the pattern section 114 of the same pattern 110. The mask 120 further comprises a rectangular phase-shifting part 123a formed closely to the blocking part 122a and three strip-shaped phase-shifting parts 123b arranged alternately in the spaces between the blocking parts 122b. The remaining area of the mask 120 is a transparent part 124.
In FIG. 1, a character xe2x80x9c0xe2x80x9d is attached to the transparent part 124, because no phase shift occurs in the exposure light passing through the part 124. A character xe2x80x9cxcfx80xe2x80x9d is attached to the phase-shifting parts 123a and 123b, because phase shift of xe2x80x9cxcfx80(180xc2x0)xe2x80x9d occurs in the exposure light passing through the parts 123a and 123b. 
The blocking part 122a has the same shape as the pattern element 113a of the circuit/element pattern 110. Each of the blocking parts 122b has the same shape as a corresponding one of the pattern elements 114a of the pattern 110.
The prior-art non-phase-shifting mask 130 in of the positive type, like the phase-shifting mask 120. The mask 130 comprises a rectangular blocking part 132 that covers the blocking parts 122a and 122b of the mask 120 and the phase-shifting parts 123a and 123b thereof. The remaining area of the mask 130 is a transparent part 134. The blocking part 132 has a following relationship with the blocking parts 122a and 122b and the phase-shifting parts 123a and 123b of the phase-shifting mask 120.
Specifically, if the non-phase-shifting mask 130 is entirely overlapped with the phase-shifting mask 120, the upper edge 132a of the blocking part 132 of the mask 130 approximately accords with the upper edges 122aa and 122ba of the blocking parts 122a and 122b of the mask 120. In this state, the upper edges 123aa and 123ba of the phase-shifting parts 123a and 123b of the mask 120 are shifted upward from the upper edge 132a of the blocking part 132 in FIGS. 1 and 2, and overlapped with the transparent part 134.
Next, a method of forming the circuit-element pattern 110 of FIG. 3 using the phase-shifting mask 120 and the non-phase-shifting mask 130 with the double exposure method is explained below.
In the first exposure step, a photoresist film (not shown), which has been formed on an object 112 for pattern formation (e.g., a polysilicon film formed on the gate dielectric film), is irradiated by specific exposure light using the phase-shifting mask 120 of FIG. 1. At this time, a latent image having the same shape as the L-shaped blocking part 122a and the linear blocking parts 122b is formed in the photoresist film thus exposed.
In the first exposure step, an undesired latent image is formed in the photoresist film thus exposed, which is due to the xe2x80x9c0-xcfx80 phase edgesxe2x80x9d formed at the locations corresponding to the edges 123aa and 123ba of the phase-shifting parts 123a and 123b. 
In the second exposure step, to eliminate the xe2x80x9c0-xcfx80 phase edgesxe2x80x9d, the photoresist film is irradiated with the same exposure light as used in the first exposure step again using the non-phase-shifting mask 130 of FIG. 2.
Thereafter, the photoresist film including the latent image is developed with a known developing solution, thereby removing the unnecessary, irradiated parts of the photoresist film. Thus, the latent image is elicited, in other words, the photoresist film is patterned as desired. As a result, the circuit/element pattern 110 of FIG. 3 is formed on the object 112 for pattern formation.
With the pattern format ion method using the above-described phase-shifting mask 120 and the non-phase-shifting mask 130, however, the following problem will occur.
Specifically, the intensity distribution of the exposure light that passes through the area of the mask 120 where the blocking parts 122b and the phase-shifting parts 123b are periodically arranged is very different from the intensity distribution of the exposure light that passes through the area of the mask 120 where the blocking part 122a and the phase-shifting part 123a are formed. It is thought that this is due to the xe2x80x9coptical proximity effectxe2x80x9d. As a result, there arises a problem that the minimum size of discriminable or formable pattern elements increases.
FIG. 4 shows the intensity change of the exposing light passing through the phase-shifting mask 120 as a function of the position, where the light intensity is shown with relative values. In FIG. 4, each of the periodically-arranged blocking parts 122b has a width of 0.1 xcexcm, each of the periodically-arranged phase-shifting parts 123b has a width of 0.2 xcexcm, the isolated blocking part 122a has a width of 0.1 xcexcm, and the isolated phase-shifting part 122b has a width of 1.6 xcexcm. The lateral axis of FIG. 4 denotes the distance from the center line of the blocking part 122a or 122b in a perpendicular direction thereto. The position xe2x80x9c0xe2x80x9d is located on the line.
As seen from FIG. 4, both the exposing light passing through the area corresponding to the isolated blocking part 122a and the isolated phase-shifting part 123a and the exposing light passing through the periodically-arranged blocking part 122b and the periodically-arranged phase-shifting part 123b have increasing intensities with the increasing distance from the center line of the part 122a. However, the increasing rate of the light intensity for the isolated parts 122a and 123a is less than that for the periodically-arranged parts 122b and 123b. Therefore, as shown in FIG. 5, if the inter-element distance (i.e., the distance between adjoining pattern elements) is larger than 0.5 xcexcm (i.e., the pattern element approaches its isolation state), the minimum, formable pattern-element size for the photoresist film increases abruptly.
Accordingly, an object of the present invention is to provide a Levenson-type phase-shifting mask that suppresses effectively the increase of the minimum, formable pattern-element size due to the above-described xe2x80x9coptical proximity effectxe2x80x9d, and a method of forming a pattern using the mask.
Another object of the present invention is to provide a Levenson-type phase-shifting mask that improves further the resolution, and a method of Forming a pattern using the mask.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the invention, a phase-shifting mask is provided. This mask is preferably used to form a pattern including at least one first pattern element and periodically-arranged second pattern elements. This mask comprises:
(a) a transparent substance;
(b) a first pattern region formed on the substrate;
the first pattern region including a first blocking part for forming at least one first pattern element;
(c) a second pattern region formed on the substrate;
the second pattern region including second blocking parts for forming second pattern elements arranged periodically;
(d) in the first pattern region, a first phase-shifting part formed at one side of the first blocking part and a first transparent part formed at an opposite side of the first blocking part to the first phase-shifting part;
(e) in the second pattern region, a second phase-shifting part formed at one side of each of the second blocking parts and a second transparent part formed at an opposite side of the second blocking part to the second phase-shifting part;
(f) a width of the first-phase shifting part in the first pattern region having a relationship with a width of the second-phase shifting part in the second pattern region in such a way that an intensity of exposing light penetrated through the first pattern region is approximately equal to an intensity of the exposing light penetrated through the second pattern region;
(g) a third blocking part formed on the substrate to surround the first phase-shifting part and the first transparent part in the first pattern region; and
(h) a fourth blocking part formed on the substrate to surround the second phase-shifting part and the second transparent part in the second pattern region.
With the phase-shifting mask according to the first aspect of the invention, the first pattern region, which includes the first blocking part for forming the at least one first pattern element, is formed on the transparent substrate. The second pattern region, which includes the second blocking parts for forming the periodically-arranged second pattern elements, is formed on same the substrate. In the first pattern region, the first phase-shifting part and the first transparent part are formed at each side of the first blocking part. In the second pattern region, the second phase-shifting part and the second transparent part are formed at each side of each of the second blocking parts. Moreover, the width of the first-phase shifting part in the first pattern region has a relationship with the width of the second-phase shifting part in the second pattern region in such a way that the intensity of exposing light penetrated through the first pattern region is approximately equal to the intensity of the exposing light penetrated through the second pattern region.
Therefore, the intensity of exposing light penetrated through the first pattern region can be approximately equal to the intensity of the exposing light penetrated through the second pattern region. This means that the xe2x80x9coptical proximity effectxe2x80x9d is effectively suppressed. As a result, the dispersion or fluctuation of the minimum pattern element size that can be formed by the mask of the invention is significantly reduced, improving the resolution.
Moreover, the third blocking part is formed on the substrate to surround the first phase-shifting part and the first transparent part in the first pattern region. The fourth blocking part is formed on the substrate to surround the second phase-shifting part and the second transparent part in the second pattern region. Accordingly, no 0-xcfx80 phase edge exists and thus, the opposite-phase interference at the 0-xcfx80 phase edges is prevented.
In a preferred embodiment of the mask according to the first aspect of the invention, the at least one first pattern element is an isolated pattern element. This is because it is advantageous that the invention is applied to such an isolated pattern element.
In this embodiment, it is preferred that the first phase-shifting part of the first pattern region has a width 0.9 to 1.4 times as large as a width of the second phase-shifting part of the second pattern region. This is because the advantages of the invention are obtainable. Moreover, it is more preferred that the first phase-shifting part of the first pattern region has a width approximately equal to a width of the second phase-shifting part of the second pattern region. This is because the advantages of the invention are most conspicuous.
In another preferred embodiment of the mask according to the first aspect of the invention, the at least one first pattern element is pattern elements periodically arranged at larger intervals (i.e., spaces) than the second pattern elements. In this embodiment, it is preferred that the intervals of the periodically-arranged pattern elements are approximately twice as large as that of the second pattern elements or larger. This is because each of the periodically-arranged pattern elements can be thought as an isolated pattern element even in this case.
In this embodiment, it is preferred that the first phase-shifting part of the first pattern region has a width 0.9 to 1.4 times as large as a width of the second phase-shifting part of the second pattern region. This is because the advantages of the invention are obtainable. Moreover, it is more preferred that the first phase-shifting part of the first pattern region has a width approximately equal to a width of the second phase-shifting part of the second pattern region. This is because the advantages of the invention are most conspicuous.
In still another preferred embodiment of the mask according to the first aspect of the invention, the third blocking part of the first pattern region and the fourth blocking part of the second pattern region are combined together. In this embodiment, there is an additional advantage that the shape of pattern elements for exposing the third and fourth blocking parts is simplified in a mask to be used in the second exposure step performed after the first exposure step that uses the mask according to the first aspect of the invention.
According to a second aspect of the invention, a method of forming a pattern is provided, in which the mask of the first aspect except of the invention is used. The method comprises:
(a) providing one of the above-described phase-shifting masks according to the first aspect of the invention as a first mask;
(b) providing a non-phase-shifting second mask;
the second mask including a transparent substrate, a fifth blocking part that covers entirely the first blocking part of the first mask, a sixth blocking part that covers entirely the second blocking part of the first mask, and a third transparent part for exposing the third and fourth blocking parts of the first mask;
(c) selectively irradiating exposing light to a photoresist film formed on an pattern-formation object using the first mask;
(d) selectively irradiating the exposing light to the photoresist film using the second mask after (c); and
(e) developing the photoresist film after (d).
With the method of forming a pattern according to the second aspect of the invention, the phase-shifting mask of the first aspect of the invention is used as the first mask for exposing the photoresist film in the first exposure process. Thereafter, the second mask having the configuration of (b) is used for exposing the same photoresist film in the second exposure process. Thus, desired pattern is formed in the photoresist film.
Thus, the xe2x80x9coptical proximity effectxe2x80x9d is suppressed effectively and as a result, the deviation of the minimum, formable pattern element size is remarkably reduced. This leads to improvement of resolution.
Additionally, the unexposed area of the photoresist film in the first exposure process using the first mask, in which the light is blocked by the third and fourth blocking parts, is exposed to the light in the second exposure process using the second mask. Therefore, no bad effect occurs even if the first mask includes the third and fourth blocking parts.
In a preferred embodiment of the method according to the second aspect of the invention, when the second mask is overlapped with the first mask, an edge of the fifth blocking part of the second mask is located in the first phase-shifting part of the first mask while an opposite edge of the fifth blocking part of the second mask is located in the first transparent part of the first mask. At the same time, an edge of the sixth blocking part of the second mask is located in the corresponding second phase-shifting part or the second transparent part of the first mask while an opposite edge of the sixth blocking part of the second mask is located in the corresponding second phase-shifting part or the second transparent part of the first mask. This is because the second mask is prevented from badly affecting in (d) the latent image formed in the photoresist film with the first mask.